mm: avoid data corruption on CoW fault into PFN-mapped VMA

[ Upstream commit c3e5ea6ee574ae5e845a40ac8198de1fb63bb3ab ]

Jeff Moyer has reported that one of xfstests triggers a warning when run
on DAX-enabled filesystem:

	WARNING: CPU: 76 PID: 51024 at mm/memory.c:2317 wp_page_copy+0xc40/0xd50
	...
	wp_page_copy+0x98c/0xd50 (unreliable)
	do_wp_page+0xd8/0xad0
	__handle_mm_fault+0x748/0x1b90
	handle_mm_fault+0x120/0x1f0
	__do_page_fault+0x240/0xd70
	do_page_fault+0x38/0xd0
	handle_page_fault+0x10/0x30

The warning happens on failed __copy_from_user_inatomic() which tries to
copy data into a CoW page.

This happens because of race between MADV_DONTNEED and CoW page fault:

	CPU0					CPU1
 handle_mm_fault()
   do_wp_page()
     wp_page_copy()
       do_wp_page()
					madvise(MADV_DONTNEED)
					  zap_page_range()
					    zap_pte_range()
					      ptep_get_and_clear_full()
					      <TLB flush>
	 __copy_from_user_inatomic()
	 sees empty PTE and fails
	 WARN_ON_ONCE(1)
	 clear_page()

The solution is to re-try __copy_from_user_inatomic() under PTL after
checking that PTE is matches the orig_pte.

The second copy attempt can still fail, like due to non-readable PTE, but
there's nothing reasonable we can do about, except clearing the CoW page.

Reported-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Tested-by: Jeff Moyer <jmoyer@redhat.com>
Cc: <stable@vger.kernel.org>
Cc: Justin He <Justin.He@arm.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Link: http://lkml.kernel.org/r/20200218154151.13349-1-kirill.shutemov@linux.intel.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/mm/memory.c b/mm/memory.c
index fcad8a0..eeae63b 100644 (file)
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2353,7 +2353,7 @@ static inline bool cow_user_page(struct page *dst, struct page *src,
        bool ret;
        void *kaddr;
        void __user *uaddr;
-       bool force_mkyoung;
+       bool locked = false;
        struct vm_area_struct *vma = vmf->vma;
        struct mm_struct *mm = vma->vm_mm;
        unsigned long addr = vmf->address;
@@ -2378,11 +2378,11 @@ static inline bool cow_user_page(struct page *dst, struct page *src,
         * On architectures with software "accessed" bits, we would
         * take a double page fault, so mark it accessed here.
         */
-       force_mkyoung = arch_faults_on_old_pte() && !pte_young(vmf->orig_pte);
-       if (force_mkyoung) {
+       if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte)) {
                pte_t entry;
 
                vmf->pte = pte_offset_map_lock(mm, vmf->pmd, addr, &vmf->ptl);
+               locked = true;
                if (!likely(pte_same(*vmf->pte, vmf->orig_pte))) {
                        /*
                         * Other thread has already handled the fault
@@ -2406,18 +2406,37 @@ static inline bool cow_user_page(struct page *dst, struct page *src,
         * zeroes.
         */
        if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE)) {
+               if (locked)
+                       goto warn;
+
+               /* Re-validate under PTL if the page is still mapped */
+               vmf->pte = pte_offset_map_lock(mm, vmf->pmd, addr, &vmf->ptl);
+               locked = true;
+               if (!likely(pte_same(*vmf->pte, vmf->orig_pte))) {
+                       /* The PTE changed under us. Retry page fault. */
+                       ret = false;
+                       goto pte_unlock;
+               }
+
                /*
-                * Give a warn in case there can be some obscure
-                * use-case
+                * The same page can be mapped back since last copy attampt.
+                * Try to copy again under PTL.
                 */
-               WARN_ON_ONCE(1);
-               clear_page(kaddr);
+               if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE)) {
+                       /*
+                        * Give a warn in case there can be some obscure
+                        * use-case
+                        */
+warn:
+                       WARN_ON_ONCE(1);
+                       clear_page(kaddr);
+               }
        }
 
        ret = true;
 
 pte_unlock:
-       if (force_mkyoung)
+       if (locked)
                pte_unmap_unlock(vmf->pte, vmf->ptl);
        kunmap_atomic(kaddr);
        flush_dcache_page(dst);